The widespread uses of the above-described, typical pressure-sensitive adhesive-coated products have accompanied increased odor problems associated with residual acrylic monomers in the solvent-type acrylic pressure-sensitive adhesives. It is accordingly desired to provide pressure-sensitive adhesive-coated products which release less odors as a result of reduced residual acrylic monomers. Furthermore, a strong demand has arisen for the reduced odor of residual acrylic monomers which results in improved working atmospheres under which pressure-sensitive adhesives are manufactured.
In the manufacture of the solvent-type acrylic pressure-sensitive adhesives, various methods have been attempted to achieve odor reduction as mentioned above, examples of which include a method which attempts to enhance a conversion by increasing catalysts in amount, a method in which a polymerization reaction period is prolonged, a method which attempts to add an increased amount of initiator in a latter stage of the polymerization reaction, and a method which involves the repeated addition of an initiator during the polymerization reaction.
However, those methods involving either adding the increased amount of initiator in the latter stage of the polymerization reaction or repeatedly adding the initiator during the polymerization reaction sometimes caused significant change in pressure-sensitive adhesive properties thereof with time, while effective in lowering the residual monomers content. In particular, among the pressure-sensitive adhesive properties, a holding power was observed to sometimes change largely with time.
Also, in Japanese Patent Laying-open No. Sho 63-175086, a method is disclosed which adds scavenger monomers after substantial completion of polymerization to reduce residual monomers. This method however resulted in substantially inadequate reaction of the scavenger monomers to leave them as residues so that it failed to reduce odors to a satisfactory extent.
While the reduced odor is pursued, an extremely reduced level of residual monomer concentration is highly sought for the medical adhesive materials utilizing the solvent-type acrylic pressure-sensitive adhesives, from additional considerations of preventing them from causing irritation, rash, itch and erythema to human body. To this end, a proposal has been made to provide a medical pressure-sensitive adhesive which, prior to introducing drugs thereto, contains therein residual monomers in an amount not to exceed 0.2 weight % of a total amount of the adhesive, on a solids content basis (Japanese Patent Laying-open No. Hei 5-131022).
A reduced degree of change in pressure-sensitive adhesion with time is highly sought for the pressure-sensitive adhesive-coated products and medical adhesive materials. In particular, for the medical adhesive materials which are applied to human body in use, such a change in pressure-sensitive adhesion with time significantly affects comfort to human body during use thereof. For example, any increase in pressure-sensitive adhesion with time may cause damages to human skin when separating the medical adhesive material therefrom. In another event where the pressure-sensitive adhesive layer increases its stiffness with time, a stress thus applied to human skin during use may cause an increased degree of irritation to human skin.
On the other hand, any reduction in pressure-sensitive adhesion with time may cause the medical adhesive material to be separated from human skin or to be partially lifted during use thereof.
In recent years, it has been highly sought that the preparations of endermic absorption type give reduced irritation to human skin during use thereof. One known method to reduce irritation to skin involves adding to the pressure-sensitive adhesive a liquid component which is compatible therewith. However, this method reduces cohesion of the pressure-sensitive adhesive to cause problems of staying or legginess thereof upon release from human skin, while effective in reducing irritation due to a plasticizing action of the liquid component.
In Japanese Patent Laying-open No. Hei 3-223212, a method is disclosed for preventing reduction in cohesion of the pressure-sensitive adhesive to relax or disperse a stress applied to human skin upon release of a medical adhesive material therefrom and thereby optimize a balance between adhesion and irritation to human skin, by introducing a liquid component into a pressure-sensitive adhesive which is subsequently coated and followed by a crosslinking treatment to form oily gels. There still remains a problem that the residue of unreacted initiators in a pressure-sensitive adhesive layer causes irritation to human skin.
Also, Japanese Patent Publication No. Hei 2-28978 discloses a method wherein a pressure-sensitive adhesive is slightly pre-crosslinked with polyfunctional monomers.
In the above case where a polymerization reaction period is prolonged to reduce residual monomers to a possible extent, a lowered productivity results. In addition, the production of gels insoluble in a solvent is encouraged to possibly allow the gels to attach to reactor walls. As a result, another problem arises which necessitates troubled operations such as for cleaning the reactor.
In particular, those methods, as described above, which either pre-crosslink the pressure-sensitive adhesive with polyfunctional monomers or effect polymerization utilizing a highly concentrated monomer solution for increasing a molecular weight of a resulting copolymer, accompany attachment of a large amount of gels onto the reactor walls. This consequently renders the operations, such as for cleaning the reactor, more troublesome to result in substantially lowered productivity and workability in the manufacture of the pressure-sensitive adhesives.
Furthermore, the formation of the above-mentioned gels in a pressure-sensitive adhesive solution causes reduced coatability of the pressure-sensitive adhesives and difficulty in obtaining preferred quality of endermic preparations.